Optical communications, remote sensing, spectroscopy, optoelectronics and imaging are just a few of the applications enabled by infrared and broadband photodetectors. In these photodetectors, a photosensitive material absorbs optical signals in the visible range and/or short wavelength infrared (SWIR) range and transforms the optical signals into electronic signals. Conventional photodetectors are typically made under vacuum processing conditions that are incompatible with high throughput, inexpensive fabrication techniques. The market penetration of the photodetectors is limited due to the high fabrication cost and/or low performance of the photodetectors. Recently, efforts have been made to address the high fabrication costs by the development of devices that can be prepared by solution processes.
Colloidal quantum dots are attractive as a material for a range of optoelectronic devices, including photodetectors, as colloidal quantum dots are solution processable, which expands the type of substrates that can be used, including integrated circuits. By their nature, quantum dots can be tuned in size to achieve a desired optical absorption spectrum. This permits the formation of thin-film photodetectors, which constitute a low cost, lightweight, flexible platform. Conventional single-crystalline semiconductors are precluded from integration with flexible electronics, particularly those including organic materials, due to the incompatibility of the processing conditions required for the semiconductors. Solutions, or suspensions, of colloidal quantum dots allow deposition using spin-coating, spray-casting, or inkjet printing techniques on virtually any substrate. Lattice mismatch considerations do not arise, and flexible substrates allow large-area processing.
Some efforts have been made to employ quantum dots in photodetectors. Konstantatos et al., Proceedings of the IEEE 2009, 97, (10), 1666-83, discloses the formation of photodetectors by the solution deposition of PbS quantum dots. Photodiodes were formed between a PbS nanocrystal film and an aluminum contact, with a planar transparent ITO thin film forming the opposing ohmic contact. MacDonald et al., Nature Materials 2005, 4, 138-42 discloses a solution processable device where a sandwich structure of glass, indium tin oxide (ITO), poly(p-phenylenevinylene) (PPV), MEH-PPV/PbS nanocrystal blend, and an upper magnesium contact is formed. In addition to acting as a hole transport layer, the PPV layer provides better electrical stability by forming a smooth and pinhole-free pre-layer on which the blend films are cast, eliminating catastrophic shorts from the upper contact directly through to the ITO, decreases the dark current by introducing an injection barrier at the ITO contact, increases the ratios of photocurrent to dark current, and permits a higher bias before electrical breakdown, resulting in a higher internal field, more efficient photogenerated carrier extraction, and higher photocurrents.